Traditional thixotropic sealants are not preferred for applying to horizontal surfaces, because the sealant will generally not form a smooth, uniform surface as it is squeezed out of the caulking tube, and customers typically prefer such smooth, uniform surfaces. Prior to curing, the surface can perhaps be smoothed over with a tool (or a finger), but this is messy, inconvenient, time-consuming, and still may not provide the desired surface appearance. Self-leveling sealants generally have a lower viscosity and therefore flow or "level out" to provide a substantially uniform and smooth, horizontal surface.
Although flow is initially desirable, consumers also want a quickly cured product, once the smooth surface is obtained. To obtain low initial viscosity which quickly increases, the art has generally used high loadings of urethane prepolymer.
In many conventional systems, the urethane prepolymer comprises free isocyanate which generally cures by reacting with ambient humidity. Such a curing reaction however generally produces carbon dioxide as a by-product. Oftentimes, the sealant surface will cure relatively quickly and trap the carbon dioxide within the sealant. In many conventional systems the carbon dioxide will concentrate toward the surfaces of the sealant and provide a substantial void cavity or cavities just under the surface of the sealant. This is often called "gassing" and is generally detrimental to the sealant's appearance and performance.
This carbon dioxide problem can be diminished by using a blocked amine curative. Such a curative will generally unblock in the presence of ambient humidity and react with the isocyanate in a reaction which does not produce an off gas. However, blocked amine curing systems are rather slow, and therefore the sealant will remain tacky longer than most consumers would like.
An alternative solution to the gassing problem is to use a two-part system, wherein the two components are mixed just prior to application. A quick cure time can thereby be achieved, but two-part systems are inconvenient, complex to use and prone to error during the mixing step. Consumers prefer one-part sealant systems.
A need therefore exists for a one-part, self-leveling formulation which has sufficient initial flow properties to provide a smooth surface, while thereafter quickly curing, but without forming concentrated voids toward the top of the cured product.